Floating roof tank and roof legs 4

[A_WAHAB]

Can the floating roof legs can take additional weight if water is present on pontoons or legs can only take the weight of floating roof?

Following is mentioned in API 650 Annex C para 3.10.2 " The legs and attachment shall be designed to support the roof and a uniform load of at least 1.2 Kpa (25 lbf/ft2. Where possible, the roof load shall be transmitted to the legs through bulkhead or diaphragms"

 

[Snickster]

A additional uniform load of 25 lb/sq ft would be a uniform water level on top of roof of 4.8 inches and would also account for other loads in combination with lesser water on roof. It would be up to the structural engineer to determine if this added design load of 25 lb/sq ft is adequate or should it be added to.

 

[Geoff13]

What is the reason for your question? What size is your tank?

You ask "... if water is present on pontoons...". The Annex C pontoons I'm familiar with have a sloped top (to drain water to the centre deck) so how could you have water on the pontoons? I'm assuming this is not the double deck roof you asked questions about in April since you mention pontoons this time.

Or do you mean you have water inside the pontoons, suggesting they have leaks, and you want to land the roof to repair it?

While Snickster is correct about the equivalent water depth, I can't think of any place you could measure this. The deck plates will sag significantly between legs such that between legs the water will be greater than 4.8" deep while directly beside a leg would be less than 4.8".

The 1.2 kPa (25 psf) is meant to allow for workers on the roof when landed on it's legs, not a flooded water loading.

 

[JStephen]

Isn't that 25 psf reduced for a roof with automatic drains? That would imply some allowance for liquid loading.

 

[HTURKAK]

I think this thread is related with the OP 's first thread ( Floating roof Diesel Tank increase in water level- 200 tons- Reason and inspection recommendaion thread1452-518655 )

With this limited info. it is hard to visualize the situation . Acc. to info provided , the upper deck is leaking and at least at some pontoons , lower deck plate is also leaking . ( the accumulation of water at bottom describe at previous post ).
If you do know the pontoons with inside water , you can calculate if the existing legs can resist. You have some allowance acc. to loading combinations applied. ( Say 50% )

If i were , i would land the roof on the legs , drain the tank and take extra safety measurements and make repair.
..

He is like a man building a house, who dug deep and laid the foundation on the rock. And when the flood arose, the stream beat vehemently against that house, and could not shake it, for it was founded on the rock..

Luke 6:48

 

[A_WAHAB]

Hi HYURKAK,

We have found water in pontoons and to lower the tank roof on its maintenance legs, I am asking whether the roof legs can withstand this extra water load? Or we have to drain the water from pontoons and then land the roof on legs

 

[HTURKAK]

[quote
Hi HYURKAK,

.... I am asking whether the roof legs can withstand this extra water load? Or we have to drain the water from pontoons and then land the roof on legs][/quote]

I do not know how many pontoons are full of water , how many is leaking . If you do know , you may calculate and see if the roof legs can withstand this extra water load. What is your plan to drain the water without compromising the safety whan the tank is operating ?

I just want to remind two points ,
- The buckling load of the roof legs have a certain FS . ( You may look to the design calculations if available ) . The load combinations give some tolerance ( say 60 % extra load )
- The double deck roof has a bending stiffness which can easily distribute the extra load to the other legs if one of leg starts buckling..

..

He is like a man building a house, who dug deep and laid the foundation on the rock. And when the flood arose, the stream beat vehemently against that house, and could not shake it, for it was founded on the rock..

Luke 6:48

 

[A_WAHAB]

Hi,

We have recently inspected Pontoons of double deck floating roof diesel tank and we have found a mixture of water and hydrocarbon inside pontoons. All pontoons have mostly found full of water with layer of diesel/hydrocarbon on the top. In addition to pontoon inspection, we have found 09 roof legs outer sleeves being sheared off (as being set on maintenance legs).

A little background, this tank was decommissioned at around 2011 and now tank has a water and diesel level of around 2.4 meters. It means roof is still floating and now we want to remove the inventory out from tank, skimmed, slip plat and put the roof on its operational/maintenance legs.

We are planning to pump out the water and hydrocarbon mixture from pontoons and then flushing the tank with water and placing on maintenance legs.

what are other possible ways to safely drain out the tank and placing tank roof on maintenance legs keeping in view 09 legs sleeves have been sheared off.

Do we need to repair the legs first after pumping out water+hydorcarbon mixture from pontoons and then placing the roof on legs so that roof does not collapse and can damage the turn dish.

 

[IFRs]

You have an interesting, dangerous and possibly deadly situation that requires careful consideration, calculation and evaluation by a group of experts in various fields (engineering, safety, operations, maintenance, qualified workers or contractors, etc). We can give you some limited assitance based on decades (or lifetimes) of experience but each situation is different and needs to be fully and exhaustively described in all facets. Only your on-the-ground team knows all the issues and actual conditions.

One thing not mentioned is that legs lose strength quickly if they are not straight and vertical. Leaning legs are much weaker than straight ones. Lean creates bending moments and a column with combined bending and axial loads has (sometimes vastly) reduced capacity.

Also, steel floating roofs have been known to spiral down, tearing up the tank bottom and killing anyone under them. Your roof has legs already broken, unexpected loads from flooding, unknown corrosion and potentially dangerous hydrocarbons floating on top of water all over the floating roof including possibly hidden or inaccessible places.

Stabilizing the roof needs to be done but you can't weld until the hydrocarbons are removed or covered. You might consider floating fire-fighting foam on the product to suppress the LELs, then weld lugs to the tank shell and FR for chainfalls to make it a safe platform for vacuum hoses and cleanup personnel, also to keep the FR from rotating and giving you a method to support and lower it. Foam is mostly water (adding weight) so this may not be a great idea, and it does not last long.

Much more needs to be known about your situation, your resources, timeline, budget, future expectations for this tank and FR, etc. For sure you need to have at least 2 or 3 experienced engineers on site to measure, document and calculate that can backup any plans with conservative calculations that pass the red-face test and potentially legal scrutiny...

 

[LittleInch]

So what is the full story here?

You have a tank "decommissioned" in 2011. Then what? It's been sitting there slowly filling up with water for 13 years to a depth of 2.4m??

Now someone wants to re-commission it?

This tank sounds like it's going to kill someone soon and may not actually be worth salvaging.

But only you can see it or have the answers.

In meant time listen to IFRS - he really does know what he's talking about.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.